If the import stalls and fails to respond after a few minutes, or it fails with an error message, you might be suffering from PHP configuration limits that are set too low to complete the process. You should contact your web host and ask them to increase those limits to a minimum as follows:

max_execution_time 120

memory_limit 256M

post_max_size 32M

upload_max_filesize 32M

You can verify your PHP configuration limits by installing a simple plugin found here.

And you can also check your PHP error logs to see the exact error being returned.